Noise, vibration and harshness reduction in a skip fire engine control system

ABSTRACT

A variety of methods and arrangements for reducing noise, vibration and harshness (NVH) in a skip fire engine control system are described. In one aspect, a firing sequence is used to operate the engine in a skip fire manner A smoothing torque is determined that is applied to a powertrain by an energy storage/release device. The smoothing torque is arranged to at least partially cancel out variation in torque generated by the skip fire firing sequence. Various methods, powertrain controllers, arrangements and computer software related to the above operations are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.14/992,779, filed on Jan. 11, 2016, which claims priority of U.S.Provisional Patent Application No. 62/102,206, filed on Jan. 12, 2015and 62/137,539, filed on Mar. 24, 2015, all which are incorporated byreference herein in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a skip fire engine control system foran internal combustion engine. More specifically, the present inventionrelates to arrangements and methods for reducing noise, vibration andharshness (NVH) using a smoothing torque that is applied to thepowertrain.

BACKGROUND

Most vehicles in operation today are powered by internal combustion (IC)engines. Internal combustion engines typically have multiple cylindersor other working chambers where combustion occurs. The power generatedby the engine depends on the amount of fuel and air that is delivered toeach working chamber and the engine speed.

The combustion process and the firing of cylinders can introduceunwanted noise, vibration and harshness (NVH). For example, the enginecan transfer vibration to the body of the vehicle, where it may beperceived by vehicle occupants. Sounds may also be transmitted throughthe chassis into the cabin of the vehicle. Under certain operatingconditions, the firing of cylinders generates undesirable acousticeffects through the exhaust system and tailpipe. Vehicle occupants maythus experience undesirable NVH from structurally transmitted vibrationsor sounds transmitted through the air. Thus, there are ongoing effortsto reduce the amount of NVH generated by internal combustion engines.

SUMMARY OF THE INVENTION

A variety of methods and arrangements for reducing noise, vibration andharshness (NVH) in a skip fire engine control system are described. Inone aspect, an operational firing fraction is generated to deliver adesired engine torque. A firing sequence is used to operate the enginein a skip fire manner. The firing sequence is based on the operationalfiring fraction. A smoothing torque is determined that is applied to apowertrain by an energy storage/release device. The smoothing torquehelps to reduce NVH generated by the skip fire firing sequence. Variousmethods, devices, powertrain controllers and computer software relatedto the above operations are also described.

Various powertrain controller designs take into account energyefficiency in selecting a firing fraction to deliver a desired torque.If the NVH can be mitigated using a smoothing torque, then some firingfractions may be used that otherwise would be unacceptable due to theirunfavorable NVH characteristics. Such firing fractions may be moreefficient than other alternatives, even taking into account the energyefficiency involved in generating a suitable smoothing torque. In someembodiments, the energy efficiency of multiple candidate firingfractions are compared and an operational firing fraction is selected.Various applications involve using a lookup table, algorithm or othersuitable mechanism to select a suitable operational firing fraction.When the engine is operated using the selected firing fraction, asmoothing torque is applied to the powertrain, if necessary, to helpreduce the resulting NVH.

The smoothing torque may be generated in a wide variety of ways. Invarious approaches, for example, the smoothing torque is based on avariation identified in the engine torque. In some applications, theengine torque can be understood to include a DC term and multipleharmonics. One or more of the harmonics (e.g., only the fundamentalfrequency, one or more of the harmonics, etc.) are selected. In someembodiments, the smoothing torque has the same frequency as the selectedharmonic(s), but its phase is offset relative to the selectedharmonic(s), e.g., the phase is shifted 180°.

In another aspect, a method will be described. A firing sequence isgenerated that is used to operate the engine in a skip fire manner. Invarious embodiments, the skip fire firing sequence is based on a shorthorizon optimal control computation. A smoothing torque is determinedthat is applied to a powertrain by an energy storage/release device. Thesmoothing torque is arranged to at least partially cancel out variationin torque generated by the skip fire firing sequence, thereby reducingNVH that would otherwise be generated by the skip fire firing sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a block diagram of a powertrain controller in a skip fireengine control system according to one embodiment of the presentinvention.

FIG. 2 is a flow chart that diagrammatically illustrates a method forreducing noise, vibration and harshness (NVH) in a skip fire enginecontrol system according to one embodiment of the present invention.

FIG. 3 is a diagram of a powertrain and a powertrain controlleraccording to one embodiment of the present invention.

FIGS. 4 and 5 are diagrams illustrating example techniques foroptimizing the reduction of NVH according to one embodiment of thepresent invention.

FIG. 6 is a graph of fuel consumed as a function of firing fractionaccording to one embodiment of the present invention.

FIG. 7 is a diagram of an engine torque waveform according to oneembodiment of the present invention.

FIG. 8 is an example diagram of a first harmonic superimposed over theengine torque waveform illustrated in FIG. 7.

FIG. 9 is an example diagram of a first harmonic.

FIG. 10 is an example diagram of a waveform including the first andsecond harmonics superimposed over the engine torque waveformillustrated in FIG. 7.

FIG. 11 is an example diagram of a second harmonic.

FIG. 12 is a block diagram of a powertrain controller in a skip fireengine control system according to one embodiment of the presentinvention.

FIG. 13 is a block diagram of an adaptive filter feed forward controlsystem according to one embodiment of the present invention.

FIG. 14 is an example waveform showing torque signatures associated withcylinder firings and cylinder skips according to one embodiment of thepresent invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to a skip fire engine control system. Morespecifically, the present invention involves methods and arrangementsfor using a smoothing torque to reduce noise, vibration and harshness(NVH) in a skip fire engine control system.

Skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, forexample, a particular cylinder may be fired during one firingopportunity and then may be skipped during the next firing opportunityand then selectively skipped or fired during the next. This iscontrasted with conventional variable displacement engine operation inwhich a fixed set of the cylinders are deactivated during certainlow-load operating conditions.

Skip fire engine control can offer various advantages, includingsubstantial improvements in fuel economy. One challenge, however, withskip fire engine control is noise, vibration and harshness. Morespecifically, there are particular firing sequences or firing fractionsthat generate substantial amounts of NVH. Such NVH is undesirable, sinceit can be felt by vehicle occupants.

One approach to dealing with such problems is to not use particularfiring fractions or firing sequences that are known to produceunacceptable NVH levels. Instead, other firing fractions or firingsequences are used and the cylinder output is adjusted accordingly(e.g., by adjusting the manifold absolute pressure, spark advance, etc.)so that the desired engine output is delivered. Various approaches ofthis kind are described in co-assigned U.S. patent application Ser. No.13/654,244, which is incorporated herein in its entirety for allpurposes.

This application describes another approach for dealing with NVH.Various implementations involve generating a smoothing torque that isapplied to a powertrain of a vehicle. The smoothing torque is any torquethat is applied to help cancel out or reduce a variation in torquegenerated by an engine. The smoothing torque can be generated by anysuitable energy storage/capture/release device. One example would be anelectric motor/generator with a battery and/or capacitor to store andrelease energy. Alternatively any system or device that stores andcaptures/releases energy mechanically, pneumatically or hydraulicallymay be used. For example, a flywheel with a variable mechanicalcoupling, or a high pressure fluid reservoir with valves controllingfluid flow to and from a turbine or similar device may be used tocapture/release energy from a powertrain. The smoothing torque isapplied in a manner such that noise and vibration generated by the skipfire firing sequence are at least partially reduced or canceled out.

In various approaches, the above smoothing torque system is appliedselectively. That is, many firing fractions and firing sequences thatdeliver a desired torque generate acceptable levels of NVH, and thus thesmoothing torque need not be applied in those circumstances. In othercircumstances, a suitable firing fraction or firing sequence maygenerate undesirable levels of NVH, but the energy expenditure, orequivalently energy costs, involved in mitigating the NVH may be toogreat. Thus, another firing fraction or firing sequence is used todeliver the desired engine output. In still other circumstances, the useof a smoothing torque may allow the use of firing fractions or firingsequence that were otherwise unacceptable due to their associated NVHlevels, and result in energy savings rather than losses. In variousembodiments, the smoothing torque system is arranged to analyze theenergy costs of the available options and select the most fuel efficientapproach that also brings NVH to acceptable levels.

Referring initially to FIG. 1, a powertrain controller 100 according toa particular embodiment of the present invention will be described. Thepowertrain controller 100 includes a firing fraction calculator 112, afiring timing determination module 120, a NVH reduction module 121, apower train parameter adjustment module 116, a sensor unit 122 and afiring control unit 140. The firing fraction calculator 112, the firingtiming determination module 120 and the NVH reduction module 121coordinate their operations to determine a suitable operational firingfraction and skip fire firing sequence for the engine. Depending on theoperational firing fraction, they may also determine a suitablesmoothing torque to reduce NVH generated by the firing sequence.

The firing fraction calculator 112 receives a torque request signal 111based on the current accelerator pedal position (APP), engine speed andother inputs. The torque request signal may be directed from the APP 163through an optional pre-processor 105 before reaching the firingfraction calculator 112. The torque request signal 111, which indicatesa request for a desired engine output, may be received or derived froman accelerator pedal position sensor or other suitable sources, such asa cruise controller, a torque calculator, an ECU, etc. An optionalpre-processor 105 may modify the accelerator pedal signal prior todelivery to the firing fraction calculator 112. However, it should beappreciated that in other implementations, the accelerator pedalposition sensor may communicate directly with the firing fractioncalculator 112.

Based on the torque request signal 111, the firing fraction calculator112 determines a skip fire firing fraction that would be appropriate todeliver the desired torque under selected engine operations and that hasacceptable NVH characteristics (with or without use of a smoothingtorque). Each firing fraction 112 is indicative of the fraction orpercentage of firings under the current (or directed) operatingconditions that are required to deliver the desired output. In somepreferred embodiments, the firing fraction may be determined based onthe percentage of optimized firings that are required to deliver thedriver requested engine torque (e.g., when the cylinders are firing atan operating point substantially optimized for fuel efficiency).However, in other instances, different level reference firings, firingsoptimized for factors other than fuel efficiency, the current enginesettings, etc. may be used in determining the firing fraction. Invarious embodiments, the firing fraction is selected from a set orlibrary of predetermined firing fractions.

The firing fraction determination process may take into account avariety of factors, including NVH, fuel efficiency and the desiredtorque. In some situations, for example, there is a particular firingfraction that delivers a desired torque in the most fuel efficientmanner, given the current engine speed (e.g., using optimized firings.)If that firing fraction is available for use by the firing fractioncalculator and also is associated with acceptable NVH levels, the firingfraction calculator 112 selects that firing fraction and transmits it tothe firing timing determination module 120, so that a suitableoperational firing sequence can be generated based on the firingfraction. The firing fraction calculator 112 also indicates to the NVHreduction module 121 that no NVH mitigation is needed, and thus theenergy/storage release device 124 does not apply any smoothing torque tothe powertrain while that sequence is used to operate the engine 150.

If the above firing fraction is instead known to generate unacceptablelevels of NVH, then the firing fraction calculator may neverthelessselect that firing fraction as the operational firing fraction. Such aselection is based on a determination that the NVH can be reduced toacceptable levels by applying a suitable smoothing torque to thepowertrain. The selection is also based on the determination that thereare no other more fuel efficient firing fraction alternatives, even whenthe energy costs associated with the NVH mitigation are taken intoaccount. In this case, the firing fraction calculator 112 transmits theselected operational firing fraction to the firing timing determinationmodule 120, so that a suitable operational firing sequence can begenerated based on the firing fraction. The firing fraction calculatoralso indicates to the NVH reduction module that mitigation of the NVH isrequired. As a result, the NVH reduction module operates the energystorage/release device 124 to apply a suitable amount of smoothingtorque on the powertrain to reduce the NVH generated by the firingsequence.

In still other circumstances, the firing fraction calculator 112 mayselect an operational firing fraction that is less ideally suited todeliver the desired torque i.e., a firing fraction that is better suitedto deliver an amount of torque that is different from the desiredtorque, but has acceptable NVH characteristics. Thus, the cylinderoutput must be adjusted (e.g., by adjusting MAP, spark advance and otherengine parameters) so that the desired torque is delivered. However, theoperational firing fraction is nevertheless more fuel efficient than theother alternatives, which may include a firing fraction with poor NVHcharacteristics where NVH mitigation is not possible or ends upconsuming too much energy.

The firing fraction calculator 112 is arranged to store and/or accessdata to help it make the above determinations and energy efficiencycomparisons. Any suitable data structure or algorithm may be used tomake the determinations. In some embodiments, for example, the firingfraction calculator 112 uses a lookup table to determine a suitableoperational firing fraction and to determine whether a smoothing torqueshould be applied. In still other embodiments, the firing fractioncalculator makes such determinations by dynamically calculating andcomparing the energy efficiency associated with different candidatefiring fractions and/or sequences. Some of these approaches will bedescribed in greater detail later in the application.

After selecting a suitable operational firing fraction, the firingfraction calculator 112 transmits the firing fraction 119 to the firingtiming determination module 120. The firing timing determination module120 is arranged to issue a sequence of firing commands (e.g., drivepulse signal 113) that cause the engine 150 to deliver the percentage offirings dictated by a commanded firing fraction 119. In someimplementations, for example, the firing timing determination module 120generates a bit stream, in which each 0 indicates a skip and each 1indicates a fire for the current cylinder firing opportunity.

The firing timing determining module 120 may generate the firingsequence in a wide variety of ways. By way of example, sigma deltaconvertors work well as the firing timing determining module 120. Instill other embodiments, the firing timing determination module selectsa suitable firing sequence from a firing sequence library based on thereceived firing fraction.

If it has been determined that there is no need to mitigate NVHgenerated by the firing sequence, the sequence of firing commands(sometimes referred to as drive pulse signal 113) outputted by thefiring timing determination module 120 may be passed to the firingcontrol unit 140 which actuates and commands the actual cylinderfirings. No smoothing torque is applied to the powertrain by the energystorage/release device 124 during the execution of the firing sequenceat the engine 150.

On the other hand, if it has been determined that the firing sequencerequires mitigation, the firing fraction calculator 112 and/or thefiring timing determination module 120 transmit the firing fractionand/or firing sequence to the NVH reduction module 121 before the firingsequence is used to operate the engine. Based on these inputs, the NVHreduction module 121 is arranged to determine a suitable smoothingtorque that would bring the NVH generated by the firing sequence toacceptable levels. In various embodiments, the smoothing torque takesthe form of one or more substantially sinusoid torque waveforms that areapplied to the powertrain and that oppose particular variations in thetorque generated by the skip fire firing sequence. The argument of thesinusoidal torque waveform may be based on the crank angle of theengine. That is, the smoothing torque can be described as smoothingtorque=sin (f*θ+φ) where θ is the crank angle, φ is the phase andf=N/4(for a V8 engine) where N is the denominator of the firing fractionlevel.

In various embodiments, the smoothing torque waveform is applied to thepowertrain by the energy storage/release device 124. The smoothingtorque involves sequentially adding torque to and then subtractingtorque from the powertrain. The timing, magnitude and pattern of thesmoothing torque may be based on a variety of factors, including but notlimited to the skip fire firing sequence, the engine speed, batterycharge or charging level in an alternative storage device, i.e.capacitor, and the current cylinder number.

It should be noted that various implementations of the smoothing torqueas described here differ from some prior art systems that used anelectric motor to fill in missing torque pulses from a skipped cylinder.Such a system requires delivering high bandwidth and amplitude torquepulses from the energy storage/release device 124. Variousimplementations of the present invention do not attempt explicitly tofill in a torque hole created by a skipped firing opportunity. Rather,such implementations consider the overall torque signature generated bya particular firing fraction or firing sequence. In theseimplementations, the control electronics seek to counter the torquevariation associated with one or more harmonics of the torque signature.Advantageously, this type of control algorithm requires a lowerbandwidth energy storage/release device 124, since the control is nottrying to cancel or mimic the high bandwidth components of the torquespike associated with a firing cylinder. Likewise, energystorage/release device 124 can deliver lower smoothing torqueamplitudes, since the system is not trying to mimic a torque spikeassociated with a firing cylinder. Both the lower bandwidth andamplitude makes the hardware associated with the energy storage/releasedevice 124 cheaper and easier to implement. A further advantage of thismethod is that lower frequency harmonics are felt more strongly byvehicle occupants, thus maximizing the reduction in sensed vibration fora given amount of smoothing torque.

Any suitable algorithm or technique may be used to generate thesmoothing torque waveform. In some embodiments, for example, the NVHreduction module 121 receives a firing sequence from the firing timingdetermination module 120, which will be later used to operate the engine150. The NVH reduction module 121 determines variations in the enginetorque that would be generated by the firing sequence. The variations inthe engine torque are responsible for the generation of NVH. Thesmoothing torque includes variations that oppose and help cancel out oneor more types of variations in the engine torque.

The characteristics of the smoothing torque may vary widely, dependingon the needs of a particular application. Consider an example process,similar to what was described above, in which the firing fractioncalculator 112 selects an operational firing fraction that is known togenerate unacceptable amounts of NVH. Thus, the NVH must be mitigated.Accordingly, the firing timing determination 120 generates a skip firefiring sequence based on the operational firing fraction, which istransmitted to the NVH reduction module 121 for analysis.

The NVH reduction module 121 determines an expected engine torque basedon the skip fire firing sequence, engine speed, firing fraction and/orany other suitable parameter. In some implementations, this enginetorque is understood to include a fixed component (i.e., a DC term) anda variable component that can be represented by multiple harmonicsinusoids, including a first harmonic (fundamental frequency) and otherharmonics. The fixed DC term propels the vehicle and the harmonics arethe unavoidable result of the variation in torque generated by aninternal combustion engine as its cylinders move through the variousstrokes of a combustion cycle. These harmonic sinusoids or variations inthe engine torque are considered to be the source of the NVH. The NVHreduction module 121 determines a smoothing torque, which is applied tothe powertrain using a particular pattern or sequence. In variousembodiments, the characteristics of the smoothing torque pattern orsequence (e.g., frequency, magnitude and phase) are designed to at leastpartially counter or oppose a selected set of one or more of theharmonic sinusoids.

An example of this concept is shown in FIGS. 7-11. FIG. 7 is a graphillustrating engine torque (N*m) applied to the crankshaft/powertrain asa function of engine angle. That is, the graph indicates a waveform 702that represents the engine torque generated by a sample skip fire firingsequence. In this example, the average torque is approximately 87 N*m.This average torque is the fixed component of the engine torque i.e.,the DC term. Using the techniques of Fourier analysis, the engine torquewaveform 702 can be expressed as the sum of this fixed component andvarious harmonics each having a fixed amplitude. (i.e., multipleharmonics including a first harmonic, second harmonic . . . tenthharmonic, etc.)

An example DC plus first harmonic waveform 802 is shown in FIG. 8,superimposed over the engine torque waveform 702. The offset firstharmonic waveform 802, has a frequency that matches the fundamentalfrequency of the waveform 702. As can be seen in FIG. 8, offset firstharmonic waveform 802 matches a significant fraction of the enginetorque waveform 702. The first harmonic component can be isolated bysubtracting the average torque (e.g., DC offset of 87 N*m) from waveform802. This results in first harmonic waveform 902 of FIG. 9. In variousembodiments, a smoothing torque waveform is generated to counter thewaveform 902 i.e., to subtract torque from the powertrain when thewaveform 902 adds torque to the powertrain, and to add torque to thepowertrain when the waveform 902 subtracts torque from the powertrain.

Various approaches involve a smoothing torque that has characteristics(e.g., frequency) that are generally identical or substantially similarto those of a selected set of one or more of the harmonic sinusoids,except that the amplitude may be different and the phase is shifted(e.g., 180°) so that the smoothing torque reduces or cancels theselected harmonic sinusoid(s). In some embodiments, the smoothing torqueis arranged to only oppose, be based on and/or have the same frequencyas the first harmonic. That is, in various embodiments, the smoothingtorque is not based on, has a different frequency from and/or does notoppose the other harmonics in the expected engine torque. The inventorshave determined that in various applications, only one or a few harmonicsinusoids need to be cancelled or reduced in order to bring NVH to anacceptable level. In the example case shown in FIGS. 7-9 the smoothingtorque can simply be configured to cancel or reduce to an acceptablelevel the first harmonic waveform 902. The smoothing torque may thushave the same frequency and amplitude as the first harmonic waveform902, but may simply be offset in phase by 180 degrees. In still otherembodiments, the smoothing torque takes into account and opposesmultiple harmonics (e.g., the first harmonic and one or more otherharmonics, etc.)

In further embodiments a DC term may be added to the smoothing torque.If the DC term is adequately large, then the smoothing torque will beuniformly in one direction; this may eliminate or reduce the impact ofany non-linear behavior (e.g. dead-zone, lash, etc) arising when aenergy storage/release device crosses zero net delivered torque. The DCterm can be in either direction, i.e. the energy storage/release devicecan store energy from the powertrain or release energy to thepowertrain. The DC term can be zero. The magnitude and sign of the DCcan depend on a number of factors including battery or capacitor chargelevel, torque demand, or other operating characteristic.

FIG. 10 illustrates a constant term and two harmonics (i.e., the firstand second harmonic) represented by waveform 1002 being isolated andsuperimposed over the example engine torque waveform 702. As can be seenby comparing FIGS. 8 and 10, the two harmonics, when combined, evenbetter match the overall variation in the engine torque waveform 702than was the case for the offset first harmonic alone in FIG. 8. FIG. 11illustrates a second harmonic waveform 1102, which represents the secondharmonic after the DC and first harmonic terms have been removed. As canbe seen by comparing FIGS. 9 and 11, the amplitude of the first harmonicwaveform 902 is substantially greater than the amplitude of the secondharmonic waveform 1102. That is, the engine torque waveform 702 has alarger first harmonic component than second harmonic component. Thelarger first harmonic component will generally generate more undesirableNVH and thus control algorithms may focus on cancelling or reducing thisharmonic component. In various applications, the smoothing torque isarranged to oppose only the first harmonic (e.g., waveform 902 of FIG.9), and not any other harmonics. It has been determined that in somedesigns, this simplifies calculation and implementation of the smoothingtorque and nevertheless is sufficient to bring NVH down to acceptablelevels. In still other embodiments, the smoothing torque is arranged tocancel or oppose multiple harmonics (e.g., a composite waveformincluding waveforms 902 and 1102). Cancelling higher harmonics may beadvantageous in reducing acoustic noise arising from the inducedvibration. For example, the first and some harmonic other than thesecond may be substantially cancelled. Specifically harmonics that arein the vicinity of the cabin boom frequency may be advantageouslysubstantially cancelled or reduced.

The magnitude (e.g., amplitude) of the smoothing torque may vary,depending on different conditions and applications. In variousembodiments, for example, the magnitude of the smoothing torque issubstantially lower than the magnitude of the engine generated harmonicsinusoid(s) that it opposes. In these embodiments, the magnitude of thesmoothing torque is arranged to reduce, not eliminate engine NVH and tobring the NVH below a predefined level that is determined to beacceptable to vehicle occupants. What defines this predefined NVH levelmay vary between different engine and vehicle designs. In variousembodiments, user testing is performed to determine the acceptable levelof NVH. Additionally, this predefined level of acceptable NVH may alsobe adjusted dynamically based on a variety of conditions, such asaccelerator pedal position, the rate of change in the accelerator pedalposition, road conditions, operating gear, vehicle speed, cabin noiselevel, presence of engine idle and any other suitable parameter. Suchconditions may be detected by one or more suitable sensors.

In some implementations, the smoothing torque is also adjusted based onfeedback received from sensor unit 122. Sensor unit 122 includes one ormore sensors that may detect a variety of engine parameters, includingbut not limited to crankshaft speed/acceleration, accelerometer data,vibration, etc. By way of example, accelerometers may be positioned at aseat rail, adjacent to, and/or inside an ECU in order to detectvibrations felt by vehicle occupants. Based on the feedback receivedfrom the sensor unit 122, the smoothing torque is dynamically adjusted.By way of example, the timing (phase) and magnitude of the smoothingtorque sequence may be changed based on the sensor input. It should beappreciated that the above sensor feedback is not required, and that invarious embodiments the smoothing torque generation system is a feedforward system.

Once the NVH reduction module prepares a suitable smoothing torque, theNVH reduction module operates the energy storage/release device 124 toapply the smoothing torque to the powertrain. The application of thesmoothing torque is coordinated with the execution of the correspondingfiring sequence at the engine 120. As a result, the smoothing torqueopposes particular variations in the engine torque, and the NVHgenerated by the skip fire firing sequence is reduced.

In the illustrated embodiment, an optional power train parameteradjusting module 116 is provided that cooperates with the firingfraction calculator 112. The power train parameter adjusting module 116directs the firing control unit 140 to set selected power trainparameters appropriately to insure that the actual engine outputsubstantially equals the requested engine output at the commanded firingfraction. By way of example, the power train parameter adjusting module116 may be responsible for determining the desired mass air charge(MAC), sparking timing, and valve timing and/or other engine settingsthat are desirable to help ensure that the actual engine output matchesthe requested engine output. Of course, in other embodiments, the powertrain parameter adjusting module may be arranged to directly controlvarious engine settings.

The firing fraction calculator 112, the firing timing determinationmodule 120, the NVH reduction module 121, the power train parameteradjusting module 116, the sensor unit 122 and the other illustratedcomponents of FIG. 1 may take a wide variety of different forms andtheir functionalities may alternatively be incorporated into an ECU, orprovided by other more integrated components, by groups of subcomponentsor using a wide variety of alternative approaches. In variousalternative implementations, these functional blocks may be accomplishedalgorithmically using a microprocessor, ECU or other computation device,using analog or digital components, using programmable logic, usingcombinations of the foregoing and/or in any other suitable manner.

Although not required in all implementations, in some implementationsdetermination of an appropriate firing fraction and/or the smoothingtorque (i.e., a determination as to whether smoothing torque will beused and what the smoothing torque will be) may be made on a firingopportunity by firing opportunity basis. That is, the currently desiredfiring fraction and/or smoothing torque can be re-determined before eachfiring opportunity based on the accelerator pedal position or otheroperating parameters. This allows the controller 100 to be particularlyresponsive to changing demands (e.g., change in the manifold absolutepressure or other engine parameters) while maintaining the benefits ofskip fire operation. In other implementations the torque generated whilechanging firing fractions can be predicted and a control system based onadaptive filters or model predictive control may be used to improve NVH.

One example where firing opportunity by firing opportunity control isadvantageous is when the desired firing fraction changes. A particularexample is if the firing fraction changes from ½ to 1. In this example,the MAP needs to be reduced to generate the right level of torque, butthis is slow, i.e. MAP has limited ability to change on a firingopportunity by firing opportunity basis. One prior art solution to thisproblem, such as described in U.S. patent application Ser. No.13/654,244, is to adjust the firing fraction at a relatively low speedto match the expected change in MAP. By constantly recalculating anddelivering an appropriate smoothing torque, the NVH reduction module canremove excessive torque resulting from a too-high MAP, allowing a fastertransition.

In some embodiments, the smoothing torque may be determined using aprecalculated future firing sequence in a short-horizon optimal controlcomputation. This control method is particularly useful when the firingsequence is non-repeating, such as during a transition between firingfraction levels. Herein short-horizon may refer to the firing decisionsthat have been made, but not yet implemented. This may be in the rangeof 4 to 20 firing opportunities, but could be more or less. Since thesedecisions are known before they are implemented the smoothing torque canbe precalculated. The smoothing torque may include both negative andpositive torques in order to obtain optimum NVH-fuel economy tradeoff,subject to motor/generator and energy storage device constraints.Motor/generator constraints may include maximum allowable torque andpower levels. Energy storage constraints may include current energystorage level and well as the maximum power transfer from the energystorage device.

Referring next to FIG. 2, a method 200 for determining a smoothingtorque according to a particular embodiment of the present inventionwill be described. Initially, at step 202, an engine torque request isreceived. In various implementations, the firing fraction calculator 112determines a desired engine torque based on the accelerator pedalposition, engine speed, a cruise controller setting and any othersuitable engine parameter.

Steps 203, 204, 206 and 208 relate to a process for evaluating differentcandidate firing fractions to select an operational firing fraction thatdelivers the desired torque and has acceptable NVH characteristics,either with or without any mitigation. In some embodiments, thepowertrain controller performs these steps as appropriate when anoperational firing fraction needs to be selected. In other embodiments,however, the evaluating of different candidate firing fractions isinstead incorporated into an algorithm, lookup table or any othersuitable decision making mechanism. That is, rather than dynamicallycomparing different candidate firing fractions on the fly, thepowertrain controller instead may consult a table or other mechanismthat directly generates the operational firing fraction based on variousinputs. In that case, the method proceeds directly to step 210.

Returning to step 203 of FIG. 2, after a desired torque level isobtained, the firing fraction calculator 112 determines whether anavailable firing fraction with acceptable NVH characteristics candeliver the desired torque while operating at optimum cylinder loade.g., the cylinder load which maximizes fuel economy. In someembodiments, for example, the firing fraction calculator 112 stores dataindicating a set of such firing fractions that are known to haveacceptable NVH characteristics while operating under optimum cylinderload under certain operational conditions. It should be appreciated thatwhich firing fractions produce acceptable NVH is a function of theengine speed and transmission gear as described in co-pending U.S.patent application Ser. Nos. 13/654,244 and 13/963,686, which areincorporated herein in their entirety for all purposes. If one of thesefiring fractions can deliver the desired torque, then the methodproceeds to step 212 and that firing fraction becomes the operationalfiring fraction.

If the firing fraction calculator 112 determines that there is no firingfraction with acceptable NVH characteristics that can deliver thedesired torque at optimum cylinder load, then the method proceeds tostep 204. At step 204, the firing fraction calculator obtains a set ofcandidate firing fractions. The set of firing fractions may include twotypes of firing fractions. One type involves one or more candidatefiring fractions with acceptable NVH characteristics that deliver thedesired torque, but only if the cylinder output is adjusted to anon-optimal load, as discussed in U.S. patent application Ser. No.13/654,244, which is incorporated herein by reference in its entiretyfor all purposes. For the purpose of this application, such a firingfraction is referred to as a “low NVH firing fraction.” The other typeof firing fraction involves one or more candidate firing fractions thatcan deliver the desired torque with less or minimal cylinder loadadjustment, but the NVH associated with such firing fractions may beunacceptable without mitigation. For the purpose of this application,such a firing fraction is referred to as a “high NVH firing fraction.”

At step 206, energy costs associated with mitigating NVH for the highNVH firing fraction(s) is/are determined. This may be performed in awide variety of ways. One example approach is described below.

In this example, the firing timing determination module 120 generates acandidate skip fire firing sequence based on the candidate high NVHfiring fraction. The torque generated by the skip fire firing sequenceand firing fraction can be modeled as a periodic waveform. Thatwaveform, in turn can, be represented as a Fourier series:

$\begin{matrix}{{{Tq}(t)} = {a_{0} + {\sum\limits_{n = 1}^{\infty}( {{a_{n}\cos \frac{n\; 2\pi \; t}{T}} + \phi_{n}} )}}} & (1)\end{matrix}$

where Tq(t) is the torque as a function of time, a₀ is the averagetorque (DC term), a_(n) is the amplitude associated with the n^(th)harmonic component, T is the period of the first harmonic (fundamentalfrequency), and φ_(n) is the phase of the n^(th) harmonic component.

Human perception of NVH varies with frequency. Typically lowerfrequencies, below approximately 8 Hz, are perceived as more annoyingthan higher frequency oscillations. The relative contribution of eachharmonic component to NVH can be defined by a weighting factor, w_(n).If w_(n) is the weight of the n^(th) harmonic, total NVH can bedetermined by taking the RMS value of the product of the weightingfunctions and the magnitude of the various harmonic frequencies:

$\begin{matrix}{{NVH} = \sqrt{\frac{1}{2}{\sum\limits_{n = 1}^{\infty}{w_{n}^{2}a_{n}^{2}}}}} & (2)\end{matrix}$

If an energy storage/release device 124 is included in the powertrain,Eq. 2 needs to be modified to include a smoothing torque applied to thepowertrain by the energy storage/release device 124. The smoothingtorque can be expressed by a Fourier expansion similar to Eq. 1 wherethe n^(th) harmonic component has a magnitude e_(n). Equation 3 belowrepresents the NVH including the effect of the smoothing torque,assuming the phase of each harmonic term of the smoothing torque isshifted by 180 degrees from the engine torque:

$\begin{matrix}{{NVH} = \sqrt{\frac{1}{2}{\sum\limits_{n = 1}^{\infty}{w_{n}^{2}( {a_{n} - e_{n}} )}^{2}}}} & (3)\end{matrix}$

The power required to create the above mitigating waveform or smoothingtorque is as follows:

$\begin{matrix}{P = {( {1 - \eta} )\sqrt{\frac{1}{2}{\sum\limits_{n = 1}^{\infty}( e_{n} )^{2}}}}} & (4)\end{matrix}$

here η is the round trip efficiency of the energy storage/releasedevice. Put another way, Equation 4 indicates the amount of energyrequired by the energy storage/release device 124 to generate thecorresponding smoothing torque. Typical values for η are 0.7 to 0.9 foran energy storage/release device based on a motor/generator andcapacitive energy storage. Other energy storage/release devices may havehigher or lower efficiencies.

It should be appreciated that Eq. 4 assumes that round trip efficiencyis constant for all harmonics and that a single energy source/sink isused. Generally these are valid assumptions as typically an internalcombustion engine is the ultimate source of all energy to drive thevehicle and only a single energy storage/release device exists withinthe vehicle. While this is generally the case, there are vehiclearchitectures where this may not be true. For example, plug in hybridsobtain energy from the electrical grid. Likewise vehicles withregenerative braking may store energy in an energy storage/releasedevice independent of the internal combustion engine. In these cases asupervisory module can access the relative costs of energy fromdifferent sources and use the optimal source or mix of sources to applythe smoothing torque. It should be noted that the round trip efficiencyof storing powertrain energy and releasing powertrain energy is alwaysless than one. The energy deficiency associated with this energytransfer can be factored in during NVH mitigation, management of theenergy level of a capacitor, from a battery, etc.

In Eqs. 3 and 4, note that the smoothing torque harmonic componentse_(n) need not have the same magnitude as their corresponding enginegenerated harmonic components a_(n). That is, the smoothing torque neednot eliminate all NVH, but instead may bring it down to a target,acceptable NVH level. At the target NVH level, the NVH may be composedof two components, NVH from harmonics that are not mitigated, i.e. thehigher harmonics and NVH from harmonics that may be incompletelycancelled.

Thus, the challenge is to determine the lowest level of energyconsumption required to reach a target, acceptable NVH. Thisoptimization problem may be expressed as a cost function captured by thefollowing equation:

$\begin{matrix}{{\min \mspace{14mu} e_{n}P} = \sqrt{\frac{1}{2}{\sum\limits_{n = 1}^{\infty}( e_{n} )^{2}}}} & (5)\end{matrix}$

subject to the following constraint:

$\begin{matrix}{{NVH}_{target} \geq \sqrt{\frac{1}{2}{\sum\limits_{n = 1}^{\infty}{w_{n}^{2}( {a_{n} - e_{n}} )}^{2}}}} & (6)\end{matrix}$

This optimization problem may be represented graphically. Two simplifiedexamples are illustrated in FIGS. 4 and 5. FIG. 4 illustrates a set ofcircles 402 a, 402 b and 402 c and a set of ellipses 404 a and 404 b,which represent the energy cost function i.e., equation (5) and theconstraint function i.e., equation (6), respectively for a particularcandidate firing fraction. This sample graph involves only the first twoharmonics. The magnitude of the first harmonic smoothing torquecomponent e₁ is given along the horizontal axis and the magnitude of thesecond harmonic smoothing torque component e₂ is give along the verticalaxis. The values of the engine generated first and second harmoniccomponents a₁ and a₂, respectively are noted. Each cylinder load, firingfraction, engine speed, and transmission gear will have an associatedset of a₁ and a₂ that can be determined through vehicle calibration orsome other means.

In FIG. 4, each concentric circle in circle set 402 a-402 c represents aconstant amount of consumed energy to mitigate torque, assuming that theefficiency of the energy storage/release device 124 is the same for thefirst and second harmonic frequencies. The smaller the circle, the lessenergy used. The center of the circle set, the origin, indicates a pointat which no energy is used, i.e. e₁=e₂=0. Each concentric ellipse in theellipse set 404 a-404 b represents a target NVH level generated by thefirst and second harmonic components. Any point on or inside the chosentarget NVH ellipse will produce an acceptable NVH level. Theeccentricity of ellipses 404 a and 404 b is determined by the ratio ofthe weighting factors w₂/w₁. For equal weighting factors, the ellipsesreduce to circles. Generally, humans are more sensitive to the lowerfrequency first harmonic and thus the ellipses are elongated verticallyin FIG. 4. Less variation is required in e₁ than e₂ to change NHV by afixed amount. The smaller the ellipse, the lower the allowed NVH. Thecenter 406 of the ellipses 404 a and 404 b represents a situation inwhich all NVH associated with the first and second harmonics has beeneliminated. At point 406, a₁=e₁ and a₂=e₂, the smoothing torque exactlycancels the first and second harmonics of the engine generated torquevariation.

To optimize energy costs, it is desirable to consume as little energy aspossible while bringing NVH down to an acceptable level. Assuming theacceptable NVH level is defined by ellipse 404 b, this goal is realizedat point A, where ellipse 404 b and the circle 402 c intersect. Point Ayields an acceptable NVH, since it is on ellipse 404 b and minimizedenergy consumption, since this point on ellipse 404 b is closest theorigin, i.e. the circle 402 c is as small as possible consistent withintersecting ellipse 404 b.

For purposes of comparison, FIG. 5 illustrates a diagram involving adifferent vehicle operating point, i.e. cylinder load, firing fraction,engine speed, and/or transmission gear. For example, the engine torque,engine speed and transmission gear may be identical to those of FIG. 4,but the firing fraction and cylinder load may be different Thisoperating point has markedly different NVH characteristics than thefiring fraction and cylinder load corresponding to FIG. 4. FIG. 5 hassimilar axes to FIG. 4 and the concentric circles 502 a, 502 b, and 502c represent constant energy expenditure from the energy storage/releasedevice 124. Similarly, ellipses 504 a and 504 b represent differentacceptable levels of NHV produced by the first and second harmoniccomponents. In FIG. 5 the engine generated first and second harmonicsare a₁ and a₂, respectively. If e₁=a₁ and e₂=a₂ the powertrain operatesat point 506 and no NVH is produced by the first and second harmonics.Assuming that the acceptable NVH level is defined by ellipse 504 b nosmoothing torque from the energy storage/release device 124 is requiredto meet the NVH target, since point B, corresponding to e₁=e₂=0, lieswithin ellipse 504 b. If ellipse 504 a represented the acceptable NVHlimit, then some smoothing torque mitigation would be required to reachthe target.

It should be appreciated that the graphical explanation shown in FIGS. 4and 5 is appropriate in the case where the first two harmonics may bemitigated by the energy storage/release device. If only the firstharmonic is considered, the two dimensional circles and ellipses wouldbecome lines. Likewise if the first, second, and third harmonics whereconsidered, the circles would become spheres and the ellipses wouldbecome ellipsoids. Generally the number of optimization variables equalsthe number of harmonics being potentially mitigated. Any number ofharmonics can be mitigated if desired, but as explained above generallyonly mitigation of one or two harmonics is required to obtain acceptableNVH performance.

The above approach assumes that an acceptable level of NVH has beenestablished. The acceptable level of NVH may be determined in anysuitable manner By way of example, extensive user testing can beperformed to determine the amount of vibration that is acceptable topassengers in a vehicle. It should be appreciated that the acceptablelevel of NVH may vary dynamically based on different conditions. In someembodiments, the acceptable level of NVH is adjusted based on roadconditions, user selection, operating gear, gear shift, vehicle speed,cabin noise level, presence of engine idle, the accelerator pedalposition (e.g., the change in rate of the accelerator pedal position)and/or any other suitable engine parameter or criteria.

Returning to FIG. 2, using any of the above techniques, the NVHreduction module 121 determines the energy cost required to mitigate theNVH of each high NVH candidate firing fraction, such that the associatedNVH is brought down to acceptable levels. The total energy costassociated with the high NVH candidate firing fraction is the sum of themitigation costs and costs associated with operating the engine at thecandidate firing fraction and cylinder load. It should be appreciatedthat any known technique may be performed to do this, and that theenergy cost estimation process is not limited to the examples, diagramsand equations provided above.

At step 208, the NVH reduction module compares the energy costsassociated with each of the candidate firing fractions. The manner inwhich this comparison is performed may vary depending on thecharacteristics of each candidate firing fraction. Consider an examplein which it is assumed that each cylinder ideally is fired under optimalconditions e.g., in which throttle position, mass air charge, sparkadvance, valve timing, and other engine parameters are substantiallyoptimized for fuel efficiency. Consider further that in this example,both a low NVH firing fraction and a high NVH firing fraction may beused to deliver the desired torque. The high NVH firing fraction is ableto deliver the desired torque under close to optimal cylinderconditions. However, mitigation is required to reduce the resulting NVH.On the other hand, the low NVH firing fraction has the oppositeproblem—while it has acceptable NVH characteristics, it cannot deliverthe desired torque without some adjustments in cylinder output i.e., bydeparting from the above optimal conditions, which results in a loss offuel efficiency. Thus, comparing the energy costs of these two candidatefiring fractions involves comparing the energy cost (losses) ofadjusting the cylinder output associated with the low NVH firingfraction with the energy cost of mitigating the NVH associated with thehigh NVH firing fraction. Such comparisons can be performed between anynumber and types of candidate firing fractions.

Based on the above analysis and/or comparisons, the NVH reduction moduleand/or the firing fraction calculator select the candidate firingfraction that delivers the desired torque in the most fuel efficientmanner (i.e., with the lowest energy cost.) In some embodiments, otherfactors are taken into account in the selection process. The selectedcandidate firing fraction becomes the operational firing fraction (step210).

An example process for selecting an operational firing fraction frommultiple candidate firing fractions is described in FIG. 6. FIG. 6 is agraph illustrating fuel consumption, inversely related to fuelefficiency, as a function of the firing fraction. The graph assumes anengine speed of 1200 RPM and an Engine Torque Fraction (ETF) of 0.2. (Inthis example, ETF represents a desired engine torque. For example, anETF=1 assumes full engine output.)

The vertical axis of the graph represents fuel consumption (grams persecond). The horizontal axis represents candidate firing fractions. Inthis figure, data points marked by a circle within a square indicate alow NVH firing fraction, in which no NVH mitigated is required to meetan acceptable NHV. Data points with an x within the circle indicate ahigh NVH firing fraction, where the NVH is unacceptable withoutmitigation. Directly above these points are points marked with a square,which indicate the total fuel consumption associated with both operatingthe internal combustion engine and smoothing the torque using theaforementioned techniques to bring the NVH to an acceptable level.

Without any NVH mitigation, point 604 represents the most fuel efficientfiring fraction selection i.e., a firing fraction of 0.5 that deliversthe desired torque, has acceptable NVH characteristics and a fuelconsumption rate of approximately 0.93 g/s. Point 606, however, is asuperior choice to point 604, because point 606 requires less energy(approximately 0.87 g/s) and uses a firing fraction of 0.4, while alsodelivering the desired torque. Although the firing fraction of 0.4 at anengine speed of 1200 is known to generate unacceptable amounts of NVH,the NVH can be mitigated using a smoothing torque. The calculated energycost of 0.87 g/s takes into account the energy costs of mitigation andyet is still less than the energy costs associated with point 604. Thus,in this simplified example, the firing fraction 0.4 is selected as theoperational firing fraction. The fuel savings in this case((0.93−0.87)/0.93) is about 6.5%, demonstrating the advantage of usingthe control method described here.

It should be noted that the selection of the operational firing factionmay be based on factors other than fuel efficiency. In some embodiments,for example, the status of the energy storage/release device 124 plays arole in the selection process. That is, consider an example in which aparticular high NVH firing fraction is determined to be suitable fordelivering the desired torque. Additionally, the NVH reduction module121 determines that the NVH associated with the firing fraction can beadequately mitigated with a smoothing torque e.g., using the techniquesdescribed above. The NVH reduction module 121 also determines the amountof energy required to generate a suitable smoothing torque. However, theNVH reduction module 121 and/or firing fraction calculator 112 may alsodetermine that the firing fraction cannot be selected as the operationalfiring fraction, because the energy storage/release device is notcurrently capable of generating the necessary smoothing torque (e.g.,based on battery status, a lack of stored energy, inability to providethe determined amount of energy, etc.) Conversely, if the energy storagedevice is nearly full, due perhaps to regenerative braking, then thecost of mitigation may be reduced compared to the prior calculation.

Returning to FIG. 2, at step 212, the firing fraction calculator 112transmits the selected operational firing fraction to the firing timingdetermination module 120. Based on the operational firing fraction, thefiring timing determination module 120 generates a skip fire firingsequence (step 212). At step 214, a determination is made as to whetherthe operational firing fraction requires NVH mitigation. If it does not(e.g., it is a low NVH firing fraction), then the method proceeds tostep 222. At step 222, the engine is operated in a skip fire mannerbased on the firing sequence.

If it is determined that the operational firing fraction does requireNVH mitigation, then the NVH reduction module 121 determines a suitablesmoothing torque (step 216). The smoothing torque may involve anysuitable smoothing torque or smoothing torque waveform that is appliedto the powertrain by the energy storage/release device 124 to helpreduce NVH generated by the firing sequence. The smoothing torque may begenerated using any suitable algorithm, technique or mechanism (e.g.,any of the techniques described in connection with FIG. 1.)

One approach may be described as follows. After the firing fractioncalculator 112 selects an operational firing fraction and determinesthat a suitable smoothing torque needs to be generated, the firingfraction calculator 112 transmits the operational firing fraction to thefiring timing determination module 120. The firing timing determinationmodule then generates a skip fire firing sequence based on theoperational firing fraction.

The firing sequence is transmitted to the NVH reduction module 121. TheNVH reduction module analyzes the skip fire firing sequence andidentifies one or more selected variations in engine torque that wouldbe generated by the sequence. This may be performed in a wide variety ofways. In some embodiments, for example, the torque can be characterizedas a torque waveform having a fixed component and a variable component(e.g., made of multiple harmonic variations/sinusoids.) Some approachesinvolve selecting the harmonic sinusoid whose frequency is thefundamental frequency. Other approaches involve selecting multipleharmonic sinusoids whose associated frequencies include the fundamentalfrequency and one or more other frequencies (e.g., the second harmonic,etc.)

The NVH reduction module 121 then generates a smoothing torque based onthe selected variations/sinusoids. As previously discussed, in variousapproaches the smoothing torque takes the form of one or more sinusoidalwaveform(s) at substantially the same frequency as the harmonicsgenerated by the internal combustion engine. In some approaches, thesmoothing torque waveform would have the same frequency as the selectedvariations, but be out of phase (e.g., offset by) 180° so as to cancelthe torque variations generated by the engine. The smoothing torque isdesigned to at least partially, but not necessarily completely, cancelout the selected variation(s), which are the source of at least someNVH. In various applications, the magnitude of the smoothing torquewaveform is designed to bring NVH generated by the firing sequence belowa predefined level.

At step 218, the engine is operated in a skip fire manner based on theoperational firing fraction selected in step 210 and its correspondingfiring sequence. At step 220, the smoothing torque determined in step216 is applied to the powertrain by the energy storage/release device124 as the skip fire firing sequence is orchestrated at the engine.Thus, the smoothing torque helps reduce the NVH generated by the skipfire firing sequence. In various embodiments, the NVH reduction module121 receives any suitable inputs (e.g., the firing sequence, the enginespeed, the current cylinder, etc.) necessary to properly coordinate theapplication of the smoothing torque and the execution of the firingsequence.

The above operations of method 200 may be performed on a firingopportunity by firing opportunity basis. Alternatively, one, some or allof the above operations may be performed somewhat less frequently, suchas one or more times per engine cycle.

Referring next to FIG. 3, a powertrain system 300 according to aparticular embodiment of the present invention will be described. Thepowertrain system 300 includes a powertrain controller 100, an internalcombustion engine 304, an energy storage/release device 124, acrankshaft 308, a transmission 312 and wheels 314. The engine 304 and/orengine storage/release device 124 are arranged to apply torque to thecrankshaft 308, which drives the wheels 314 through the transmission312. The powertrain controller, which is described in FIG. 1, isarranged to coordinate the operation of the engine 304 and the energystorage release device 124. The powertrain system may be operated usingany of the techniques described in connection with FIGS. 1, 2, 12, and13. It should be appreciated that although a particular powertrainconfiguration is illustrated in FIG. 3, the components of the figure maybe positioned in any suitable arrangement.

The energy storage/release device 124 is arranged to add torque to orsubtract torque from the powertrain. In various embodiments, the energystorage/release device 124 generates a smoothing torque pulse waveform.The smoothing torque pulse waveform applied by the energystorage/release device 124, may be substantially a sum of one or moresinusoidal waveforms applying torque at one moment and subtractingtorque at another moment. Generally, the smoothing torque pulse waveformis arranged to cancel a selected variation of torque generated by theengine (e.g., as discussed in connection with the NVH reduction module121 of FIG. 1 and step 216 of FIG. 2.)

The energy storage/release device 124 may be any suitable device ordevices that can absorb or subtract torque from the powertrain, storethe resulting energy, and/or use the energy to add torque to thepowertrain. In various implementations, the energy storage/releasedevice 124 includes a motor/generator and a battery or a capacitor. Inother implementations, the energy/storage release device 124 stores andreleases energy mechanically (e.g., a flywheel), pneumatically orhydraulically.

Some embodiments involve an energy storage/release device 124 that isarranged to have multiple applications i.e., other applications inaddition to generating a smoothing torque. In some applications, forexample, the energy storage/release device 124 also subtracts torquefrom and adds torque to the powertrain in the same manner as any modernhybrid vehicle in order to improve fuel efficiency (e.g., usingregenerative braking, etc.). That is as well as supplying an oscillatingsmoothing torque the energy storage/release device supplies a DCcomponent to the powertrain torque. This DC component may be positive ornegative depending on the operating conditions, the amount of energycurrently stored in the energy storage/release device and othervariables. The DC component may be chosen in part to compensate for theinefficiencies associated with storing and releasing energy from theenergy storage/release device. The energy storage/release device 124 mayalso be an integrated starter-generator used to restart an engine aspart of a start/stop engine system.

In various approaches, the energy storage/release device 124 is alsoused to smooth transitions between different firing fractions. By way ofexample, if the engine is operated in a skip fire manner and shiftingfrom a lower firing fraction to a higher firing fraction, the manifoldabsolute pressure may take time to adjust from a higher to a lowerlevel. That is, if the shift is made immediately, the vehicle may leapforward because the cylinder output will be too great. In variousapplications and under such circumstances, the energy storage/releasedevice 124 is arranged to absorb/supply torque from/to the powertrain,thereby helping to ensure a smoother transition between the firingfractions. By way of example, the powertrain controller 100 and theenergy storage/release device 124 may be operated using any of thetechniques or operations described in U.S. patent application Ser. No.13/654,244 and U.S. Provisional Patent Application No. 62/053,351, whichare incorporated by reference in their entirety for all purposes.

The invention has been described primarily in the context of controllingthe firing of 4-stroke piston engines suitable for use in motorvehicles. However, it should be appreciated that the described skip fireapproaches are very well suited for use in a wide variety of internalcombustion engines. These include engines for virtually any type ofvehicle—including cars, trucks, boats, construction equipment, aircraft,motorcycles, scooters, etc.; and virtually any other application thatinvolves the firing of working chambers and utilizes an internalcombustion engine. The various described approaches work with enginesthat operate under a wide variety of different thermodynamiccycles—including virtually any type of two stroke piston engines, dieselengines, Otto cycle engines, Dual cycle engines, Miller cycle engines,Atkinson cycle engines, Wankel engines and other types of rotaryengines, mixed cycle engines (such as dual Otto and diesel engines),radial engines, etc. It is also believed that the described approacheswill work well with newly developed internal combustion enginesregardless of whether they operate utilizing currently known, or laterdeveloped thermodynamic cycles.

In some preferred embodiments, the firing timing determination moduleutilizes sigma delta conversion. Although it is believed that sigmadelta converters are very well suited for use in this application, itshould be appreciated that the converters may employ a wide variety ofmodulation schemes. For example, pulse width modulation, pulse heightmodulation, CDMA oriented modulation or other modulation schemes may beused to deliver the drive pulse signal. Some of the describedembodiments utilize first order converters. However, in otherembodiments higher order converters or a library of predetermined firingsequences may be used.

It should be appreciated that the powertrain controller designscontemplated in this application are not limited to the specificarrangements shown in FIGS. 1 and 3. One or more of the illustratedmodules may be integrated together. Alternatively, the features of aparticular module may instead be distributed among multiple modules. Thecontroller may also include additional features, modules or operationsbased on other co-assigned patent applications, including U.S. Pat. Nos.7,954,474; 7,886,715; 7,849,835; 7,577,511; 8,099,224; 8,131,445; and8,131,447; U.S. patent application Ser. Nos. 13/774,134; 13/963,686;13/953,615; 13/886,107; 13/963,759; 13/963,819; 13/961,701; 13/963,744;13/843,567; 13/794,157; 13/842,234; 13/004,839, 13/654,244; 13/004,844;14/207,109; and 13/681,378 and U.S. Provisional Patent Application Nos.61/952,737 and 61/879,481, each of which is incorporated herein byreference in its entirety for all purposes. Any of the features, modulesand operations described in the above patent documents may be added tothe controller 100. In various alternative implementations, thesefunctional blocks may be accomplished algorithmically using amicroprocessor, ECU or other computation device, using analog or digitalcomponents, using programmable logic, using combinations of theforegoing and/or in any other suitable manner.

A technique for reducing vibration using an electric machine isdescribed in U.S. Pat. No. 8,015,960, although the technique differsfrom various embodiments of the present invention in several respects.For one, the '960 patent focuses on a variable displacement enginecontrol system, not a skip fire engine control system. Also, the '960patent describes the following process: 1) determining torque applied toa crankshaft; 2) extracting components from the torque attributable touneven cylinder firings in variable displacement mode; 3) removing afixed component (a fixed target torque) from the extracted component toleave only the variable component in the extracted components; 4)generating a vibration-damping torque that opposes the variablecomponent generated in the third step. In other words, thevibration-damping torque is arranged to oppose all variation (i.e.,after removal of the target torque) attributable to uneven cylinderfirings. In various embodiments of the present invention, a smoothingtorque is generated that does not necessarily oppose all variationattributable to uneven cylinder firings. Rather, in some embodiments,the smoothing torque opposes only some of the variation (e.g., thesmoothing torque may oppose only one or more of the harmonic sinusoidswith particular frequencies, such as the fundamental frequency, etc.) Invarious implementations, the smoothing torque does not oppose particulartypes of variations attributable to uneven cylinder firings e.g., maynot oppose one or more other harmonic sinusoids. Various embodiments ofthe present invention also describe a control algorithm that appliesonly a sufficient amount of smoothing torque to meet an NVH target. Thecontrol algorithm also selects an operational firing fraction thatmaximizes fuel efficiency considering the energy costs associated withgenerating the smoothing torque. Another distinction between the currentinvention and the prior art is that, the frequency of the smoothingtorque may not be equal to the firing frequency. For example, at 1500RPM and a firing fraction of 40%, the firing frequency is 40 Hz, but thedesired smoothing torque may have a frequency of 20 Hz.

While the invention has been generally described in terms of using afiring fraction to characterize the firing sequence, this is not arequirement. FIG. 12 shows an embodiment of a power train controller1200. Many of the various elements of power train controller 1200 aresimilar or identical to those shown and described in relation to powertrain controller 100 shown in FIG. 1. Unlike power train controller 100,FIG. 12 shows the drive pulse signal 113 generated directly from atorque request signal 111 without reference to a firing fraction.Instead a firing sequence generator 1202 may produce drive pulse signal113. Drive pulse signal 113 may consist of a bit stream, in which each 0indicates a skip and each 1 indicates a fire for the current cylinderfiring opportunity that defines the firing sequence. The firing decisionassociated with any firing opportunity is generated in advance of thefiring opportunity to provide adequate time for the firing control unit140 to correctly configure the engine, for example, deactivate acylinder intake valve on a skipped firing opportunity. Each firingopportunity will have a known torque signature depending on whether thefiring opportunity corresponds to a skip or a fire and the settings ofthe power train parameters defined by power train parameter adjustingmodule 116.

The firing sequence and smoothing torque may be determined using avariety of methods. In one embodiment short horizon model predictivecontrol, which includes matching of the requested and delivered torque,NVH, and energy costs associated with producing the smoothing torque aspart of the optimization problem may be used. In various embodiments,model predictive control is an optimal control method whichstraightforwardly handles systems with multiple performance criteriausing a short horizon optimal control computation. In variousimplementations of this method a discrete optimization is performed ateach time using new system measurements to compute the best systeminputs to apply to the controlled system at the current time. The methodrepetitively solves this optimization each time a new input is desired.Inputs to the model may include the requested torque, the torquesignature associated with skips and fires, acceptable NVH level,acceptable emission level, and energy costs and energy/power constraintsassociated with generation of a smoothing torque. Model variables mayinclude, but are not limited to, engine speed, transmission gearsetting, engine and ambient temperature, road conditions and engineparameters, such as MAP, valve timing, and spark timing.

Applying this control method may involve various combinations of powertrain parameters, smoothing torques and firing sequences that deliverthe requested torque being determined and evaluated at each firingopportunity in the firing sequence generator 1202. The firing sequencegenerator 1202 may then produce a firing sequence that deliversoptimum/improved fuel economy with acceptable NVH subject to the systemconstraints. This control method is particularly useful when the firingsequence is non-periodic, such as during a transition between firingsequences associated with changing torque requests 111 but also appliesnaturally to steady torque requests as well. Here short horizon mayrefer to firing decisions that have been made, but not yet implemented.This may be in the range of 4 to 8 firing opportunities, since thesedecisions are known before they are implemented, the smoothing torquecan be precalculated. Constraints on the smoothing torque may includemaximum allowable torque levels and frequency delivery limitation.Energy storage constraints may include current energy storage level andwell as the maximum power transfer from the energy storage device.

In another embodiment adaptive filter (AF) feed forward (FF) control maybe used to attenuate undesired torque oscillation caused by combustionevents. In some embodiments AF-FF control can take advantage of the factthat the firing sequence and resultant torque disturbances are clearlydefined. FIG. 13 shows a schematic diagram of AF-FF control. Theobjective of AF-FF control is to attenuate a disturbance on a system ofinterest, and AF-FF control achieves this objective by generating adisturbance cancelling signal which counteracts the disturbance whenapplied to the system. In this case the disturbance is the variation inthe engine torque 1310 from its mean value. A filter output 1314 isgenerated by a digital filter 1304 based on an inputted disturbancecorrelated signal 1312. The disturbance correlated signal 1312 may havea mean value of zero, so as not to alter the average overall power trainoutput torque. The disturbance correlated signal 1312 containsinformation regarding the expected disturbance with some time advance.This signal 1312 may be based on a firing sequence and estimated torquesignatures associated with skips and fires. The firing sequence may bederived using a torque request, firing fraction, sigma-delta filter, alook up table, a state machine or by some other means. The filter output1314 may be inputted to a smoothing torque module 1302, which generatesa smoothing torque 1316. The smoothing torque module 1302 represents thedynamic response of a motor/generator or any other system that suppliesa smoothing torque which includes any response delays or limitation soas to generate the desired smoothing torque 1316. The smoothing torque1316 is combined with the engine torque 1310 in summing junction 1318.Summing junction 1318 outputs a delivered torque 1320 to the powertrain. The summing junction 1318 shows the smoothing torque 1316 beingsubtracted from the engine torque 1310. It should be appreciated that inother embodiments the smoothing torque may have the opposite polarityand the smoothing torque is added to the engine torque.

Adaptive filter parameters called weights may be updated by a weightupdate module 1306 that uses an adaptive algorithm to minimizedifferences between the smoothing torque 1316 and the disturbance, theengine torque 1310 less the mean value, so as to smooth the deliveredtorque 1320. The weight update module 1306 uses a model of smoothingtorque module 1302 and inputs of both the delivered torque 1320 anddisturbance correlated signal 1312 to determine the appropriate weights.The minimization may involve minimizing a mean square difference betweenthe signals, although other minimization criteria may be used.

Graphs depicting the time behavior of the various signals in FIG. 13 areshown in order to better understand and explain operation of examplepower train controller 1300. The engine torque graph 1311 depictsoscillations in the engine torque output similar to those previouslyshown in FIG. 8. The disturbance correlated signal graph 1313 shows anestimated signal of the disturbance in engine torque which will benecessary to minimize variations in the delivered torque 1320. Thisestimate reflects the skip fire nature of the firings, so it willprovide the necessary frequency component information to the digitalfilter 1304, which will result in the filter output 1314 having theproper frequency components. Based on the various weights associatedwith magnitude and phase response of the filter, the digital filter 1304will adaptively control the filter output 1314 so as to minimizedisturbances in delivered torque 1320. Filter output graph 1315illustrates how the disturbance correlated signal 1312 is modified bydigital filter 1304. The filter output 1314 is inputted into thesmoothing torque module 1302, which includes a motor/generator or somesimilar system that can generate or absorb torque. The smoothing torquemodule 1302 outputs a smoothing torque 1316 depicted in graph 1317.Graph 1317 illustrates how the smoothing torque 1316 matches and cancelsthe variations in the engine torque 1310. When the smoothing torque andengine torque are combined in adder 1318 the resultant delivered torque1320 has relatively small torque variations as depicted in graph 1321.

An advantage of various implementations of AF-FF control is that sinceit is a feed forward control, it can eliminate or at least minimize anydisturbance in the disturbance correlated signal within the bandwidth ofthe smoothing torque module, provided the time advance of thedisturbance correlated signal 1312 with respect to the disturbance islarger than the delay caused by the torque smoothing module 1302 andfilter computations in digital filter 1304. Adaptation of the weightsused in digital filter 1304 is relatively slower than the change ofdisturbance, but it does not restrict the ability of the controller 1300to attenuate the disturbance. The adaptive algorithm determines thepower train characteristics relating the disturbance correlated signal1312 and the actual disturbance (variation in engine torque 1310), whichare fixed or whose change rate is much slower than that of thedisturbance.

One input into both adaptive filter feed forward and short horizon modelpredictive control is the torque signature associated with cylinderskips and fires. FIG. 14 shows representative torque signaturesassociated with fires curve 1410 and no fires (skips) curve 1420. Theserepresentative curves depict the normalized torque output associatedwith a cylinder through a working cycle of 720 degrees of crankshaftrotation. These representative torque signatures can be scaled andadjusted based on the engine parameters. The total engine torque isgiven by the sum of the torque generated by all cylinders. The totalestimated engine torque may then be used in as part of a short horizonpredictive model control or an adaptive filter feed forward controlsystem.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, the drawings and the embodiments sometimesdescribe specific arrangements, operational steps and controlmechanisms. It should be appreciated that these mechanisms and steps maybe modified as appropriate to suit the needs of different applications.For example, some or all of the operations and features of the NVHreduction module are not required and instead some or all of theseoperations may be transferred as appropriate to other modules, such asthe firing fraction calculator and/or the firing timing determinationunit. Additionally, although the method illustrated in FIG. 2 implies aparticular order, it should be appreciated that this order is notrequired. In some embodiments, one or more of the described operationsare reordered, replaced, modified or removed. While the invention isapplicable to all forms of hybrid vehicles it is particularly applicableto micro-hybrids, which have relatively small energy storage andmotor/generator capacity insufficient to provide the entire motive forceto drive the vehicle. The invention is also applicable to engines havingany number of cylinders. Various embodiments of the invention areparticularly advantageous in compact vehicles having relatively smallengines, such as 2, 3 or 4 cylinder engines, where the NVH associatedwith a low cylinder count, skip fire engine can be mitigated by asmoothing torque. Therefore, the present embodiments should beconsidered illustrative and not restrictive and the invention is not tobe limited to the details given herein.

What is claimed is:
 1. A powertrain controller for directing operationof an internal combustion engine having a plurality of cylinders, eachcylinder being arranged to operate in a sequence of working cycles, eachworking cycle having an associated firing opportunity, the internalcombustion engine having an associated engine cycle that constitutes onefiring opportunity for each cylinder, the powertrain controllercomprising: a firing controller arranged to direct a sequence ofcylinder working cycle firings, wherein selected first working cyclesare directed to have a greater torque output than selected secondworking cycles within the same engine cycle, the first and secondworking cycles being interspersed; and an NVH reduction module that isarranged to determine a smoothing torque that is applied to a powertrainby an energy storage/release device wherein the smoothing torque isarranged to at least partially cancel out a variation in torque outputgenerated by the first and second working cycle firings.
 2. A powertraincontroller as recited in claim 1 wherein: the firing sequence generatesa torque that includes a plurality of harmonic frequencies, one of theharmonic frequencies being a fundamental frequency; and the smoothingtorque is periodic with a frequency that is substantially the same asthe fundamental frequency.
 3. A powertrain controller as recited inclaim 1 wherein the smoothing torque includes only a single frequency.4. A powertrain controller as recited in claim 1 wherein the NVHreduction module is further arranged to select a magnitude of thesmoothing torque based on fuel efficiency and further based on whetherthe smoothing torque brings NVH generated by the operation of the enginebelow a predefined NVH target level.
 5. A powertrain controller asrecited in claim 1 wherein the energy storage/release device utilizes atleast one selected from the group consisting of a capacitor, a battery,a flywheel, a pressurized pneumatic reservoir, and a pressurizedhydraulic reservoir.
 6. A powertrain controller as recited in claim 1wherein the energy storage/release device utilizes at least one selectedfrom the group consisting of an integrated starter, a motor/generator, acontrollable mechanical coupling, and a turbine.
 7. A powertraincontroller as recited in claim 1 further comprising a firing fractioncalculator, wherein the firing fraction calculator and the NVH reductionmodule are arranged to: select the operational firing fraction from aset of available firing fractions that includes a first firing fractionthat is associated with unacceptable NVH levels and a second firingfraction that is associated with acceptable NVH levels; determine energycosts associated with mitigating the NVH levels associated with thefirst firing fraction, the mitigating of the NVH levels involvingapplying the smoothing torque to the powertrain to help reduce the NVHlevels associated with the first firing fraction wherein the selectionof the operational firing fraction is based at least in part on theenergy costs determination.
 8. A powertrain controller as recited inclaim 7 wherein the NVH reduction module is further arranged to:determine energy costs associated with the second firing fraction, whichis associated with acceptable NVH; compare energy costs associated withthe first and second firing fractions; and based on the comparison,select one of the set of available firing fractions as the operationalfiring fraction.
 9. A powertrain controller as recited in claim 1wherein the NVH reduction module is further arranged to: receive sensordata that includes one selected from the group consisting of 1)crankshaft acceleration sensor data; 2) accelerometer data received fromone or more accelerometers mounted on a vehicle; and 3) accelerometerdata received from one or more accelerometers mounted in an enginecontrol unit (ECU); and adjust the smoothing torque based on the sensordata.
 10. A powertrain controller as recited in claim 1 wherein thedetermination of the smoothing torque is performed on a firingopportunity by firing opportunity basis.
 11. A powertrain controller asrecited in claim 1 wherein the determination of the smoothing torque isbased on adaptive feed forward control.
 12. A method for operating aninternal combustion engine having a plurality of cylinders, eachcylinder being arranged to operate in a sequence of working cycles, eachworking cycle having an associated firing opportunity, the internalcombustion engine being part of a powertrain and having an associatedengine cycle that constitutes one firing opportunity for each cylinder,the method comprising: generating a sequence of cylinder working cyclefirings, wherein selected first working cycles are directed to have agreater torque output than selected second working cycles within thesame engine cycle, the first and second working cycles beinginterspersed; determining a smoothing torque, wherein the smoothingtorque is arranged to at least partially cancel out variation in torquegenerated by the first and second working cycle firings; and applyingthe smoothing torque to the powertrain by an energy storage/releasedevice.
 13. A method as recited in claim 12, further comprising:obtaining a set of available firing fractions that can deliver thedesired torque; determining a first subset of one or more of theavailable firing fractions that have acceptable NVH levels; determininga second subset of one or more of the available firing fractions thathave unacceptable NVH levels; and determining an energy cost associatedwith NVH mitigation required to reduce NVH levels for each firingfraction in the second subset to acceptable levels wherein the NVHmitigation involves applying the smoothing torque to the powertrain toreduce vibration generated by the skip fire firing sequence.
 14. Amethod as recited in claim 13, further comprising: determining totalenergy costs for each of the second subset of firing fractions, whereinthe total energy costs include costs associated with both theoperational firing fraction and mitigation; determining energy costs foreach firing fraction in the first subset of firing fractions; comparingthe energy costs of the first and second subsets of firing fractions;and based on the comparison, selecting the operational firing fractionfrom the first and second subsets of firing fractions.
 15. A method asrecited in claim 12 further comprising: receiving sensor data thatincludes one selected from the group consisting of 1) crankshaftacceleration sensor data; 2) accelerometer data received from one ormore accelerometers mounted on a vehicle; and 3) accelerometer datareceived from one or more accelerometers mounted in an engine controlunit (ECU); and adjusting the smoothing torque based on the sensor data.16. A method as recited in claim 12 wherein the determination of thesmoothing torque is performed on a firing opportunity by firingopportunity basis.
 17. A method as recited in claim 12 wherein thedetermination of the smoothing torque is dependent on the state of theenergy storage/release device.